During DC operation of a High Intensity Discharge lamp, it is often noticed that without stabilization of the arc, the anode tends to overheat. This is due to the collection of energy from the incoming electrons. Other cataphoretic problems that have been reported during operation with unidirectional current include damage to the anode, wall blackening, chemical separation, color separation, and shortened operating life of the lamp.
To prevent these problems from occurring, one method embodiment of the present invention introduces a lamp-shorting pulse to modulate the power waveform. The modulated waveform creates conditions, which overcome the cataphoretic forces on the easily ionized atoms. In addition to correcting the detrimental cataphoresis and anode overheating effects during operation, the inventive method also corrects for convective forces by operating in a constrictive mode. These corrections allow the lamp to be operated in either a vertical or a horizontal burning position without adverse arc bowing effects.
The method of this invention modulates the input power to a HID lamp. This modulation eliminates DC cataphoretic effects. A unidirectional power waveform is utilized in which the amplitude is modulated, e.g., with a lamp shorting pulse. In addition, this invention utilizes unidirectional current ballasts which can be produced with fewer components with simplified topologies, and at a lower cost.
The chemical species separation and, therefore, color separation of a typical 100 watt metal halide (MH) lamp is normally very apparent after only a few minutes of standard DC operation. With the modulation method of this invention, color distortion is not noticeable even after several hours of operation.